High power transistor base drive circuit

ABSTRACT

In a base drive circuit for a high power transistor, an external signal is applied to the circuit by an optical coupler. In response to the external signal, the circuit provides a positive current to the high power transistor in order turn it on. The positive constant current is characterized by an initial spike that prevents any localized hot spots from developing in the high power transistor and thereafter the positive current becomes constant. The circuit also provides a constant negative current when the external signal is removed, thereby quickly turning off the high power transistor. The optical coupler and a time delay device provide the circuit with high noise immunity.

Recently, high speed, power transistors have become available with thecapability of, for example, switching power in the range above 20,000watts in less than 5 microseconds. An example of such a transistor isthe KD22457510, made by Westinghouse Electric Co. Although thesetransistors have been designed to potentially provide fast switchingcapability, the high speed operation often cannot be realized in manyapplications, particularly, if false triggering is to be avoided andfault protection is to be provided.

One of the problems encountered with the fast switching transistors isthe increased sensitivity to noise and spurious triggering signals.Slower transistors, due to their slow response to input signals, have acorresponding immunity from short noise signals. The faster switchingtransistors, however, can respond to relatively short spurious noisepulses and, hence, have a much greater noise sensitivity.

These problems cannot be overcome by simply reducing the drive currentto reduce the switching speed since, during the initial turn-on of thetransistor, this can result in a non-uniform current spread throughoutthe die resulting in undesirable concentration of high current and hotspots. Attempts at solving this problem by simply increasing the basecurrent drive to a higher level to provide a faster turn-on can, on theother hand, easily destroy the transistor under partially conductivefault conditions.

An object of this invention is to provide a base drive circuit for highpower, fast switching transistors, that permits effective operation athigh switching rates.

Another object of the invention is to provide a base drive circuit forpower transistors having a high noise immunity in both the on and offconditions.

A still further object of the present invention is to provide negativebase current for turning off quickly the power transistor when faultsoccur or for turning off quickly the power transistor to minimizeswitching times and switching losses during normal switching.

Still another object of the invention is to provide a base drive circuitfor power transistors with a fast turn-on capability that avoidsformation of hot spots in the transistor and which also protects thetransistor during partially conductive fault conditions.

SUMMARY OF THE INVENTION

The transistor base drive current for the power switching transistormust be sufficient in the "on" condition so as to maintain thetransistor in the saturated or fully conductive state throughout theentire range of load currents being supplied by the transistor. A highbase current is also desirable during the turn-on period so that thecurrent is supplied quickly and spreads uniformly throughout the die toeliminate concentration of currents and consequent hot spots. On theother hand, high base current can easily destroy the transistor underfault conditions that require high power dissipation under partiallyconductive conditions. With the base drive circuit according to theinvention the base current is supplied by a constant current regulatorwith an initial, short interval, high current override. In the quiescent"on" state, the constant current regulator maintains the powertransistor base current at a level high enough to maintain thetransistor in saturation throughout the range of anticipated loadcurrents but, at the same time, low enough to prevent high power lossdamage under partially conductive fault conditions. The initial overrideprovides a sufficiently high current to the base of the power transistorto force a rapid current spread over the die to avoid hot spots andprovide rapid turn-on. The period of time during which initial highcurrent is present, however, is sufficiently short that, even should afault exist, the power dissipation is insufficient to destroy thetransistor. With this base drive arrangement, noise immunity is providedusing an optical coupler in combination with a time delay circuit toreduce sensitivity to spurious noise signals which might otherwise turnthe transistor on. A continuous reverse current base drive is utilizedto provide noise immunity during the off condition. The reverse basedrive not only provides noise immunity, but also provides rapidturn-off, and as a consequence, reduces power losses under faultconditions to thereby add additional protection for the powertransistor.

GENERAL DESCRIPTION OF THE DRAWINGS

The manner in which the foregoing and other objects are achievedaccording to the invention will become clear from the followingspecification which includes the drawings and wherein:

FIG. 1 is a schematic diagram illustrating the base drive circuitaccording to the invention;

FIG. 2 is a curve illustrating the character of the base drive current.

DETAILED DESCRIPTION OF THE INVENTION

The base drive circuit for a fast switching power transistor 10 is shownin FIG. 1 including an isolation and time delay circuit 14, a turn-oncircuit 16 and turn-off circuit 18. The isolation and time delay circuitincludes an optically coupled input circuit, which isolates the basedrive circuit form the external circuits, and a time delay circuitdesigned to reduce sensitivity to short spurious noise signals. Turn oncircuit 16 is a constant current regulator which normally provides apre-determined constant current to the base of transistor 10 at a levelsufficient to maintain the power transistor in saturation under allanticipated load conditions and, at the same time, insufficient topermit damage to the transistor under partially conductive conditionswhich may occur during faults. The turn-on circuit further includes acurrent override to provide a short initial high current to rapidlyturn-on the transistor so as to avoid high current concentrations in thetransistor die which may create hot spots. Turn-off circuit 18 providesa continuous reverse current to the base of the power transistor 10 torapidly turn off the transistor and to reduce noise sensitivity in theoff state.

The base drive circuit is powered from two isolated power supplies 20and 21 which provide a positive and a negative source, respectively. Thepower supplies include transformers 24 and 25 having primary windingsconnected to an AC source and secondary windings connected to diodebridges 26 and 27, respectively, arranged in full wave bridgeconfigurations. Filter capacitors 28 and 29 are connected across theoutputs of the diode bridges. The negative plate of capacitor 28 and thepositive plate of capacitor 29 are connected to the common for the basedrive circuit. Thus, power supply 20 on the other plate of capacitor 28provides a positive source whereas power supply 21 on the other plate ofcapacitor 29 provides a negative source.

Isolation and time delay circuit 14 includes an optically coupled inputcircuit followed by a time delay circuit which reduces sensitivity toshort noise signals. The input circuit includes a light emitting diode(LED) 31 in a light tight enclosure 30 with a photo sensitive transistor32. LED 31 is connected to the external circuit and receives the signalsfor controlling the state of the power transistor 10. Current flowthrough diode 31 causes the diode to emit light which, in turn, renderstransistor 32 conductive. The base of transistor 32 is connected to thepositive source via a diode 33. The collector of transistor 32 isconnected to the positive source via resistors 35 and 36 in serieswhereas the emitter of the transistor is connected to the common. Thus,current flow through light emitting diode 31 renders transistor 32conductive which in turn causes current flow through resistors 35 and 36developing a potential drop across the resistors.

The main timing element of the time delay circuit is capacitor 40. Thecharging circuit for the capacitor includes the emitter-base circuit ofa PNP transistor 41 connected in series with a resistor 42 between thepositive source and ground. A diode 43 is connected across resistor 42and is poled in a direction to provide a discharge path for capacitor 40via resistor 44 connected between the collector of transistor 41 and thecommon. A voltage threshold circuit 45, such as integrated circuit 555N,is connected across capacitor 40 and responds to the voltage across thecapacitor. When the potential supplied to the input of voltage thresholdcircuit 45 reaches approximately two thirds of the supply voltage, thethreshold circuit turns on and the output thereof goes from high tozero.

When the potential across resistor 36 becomes sufficiently high due tothe signal applied to LED 31 and the conductivity of transistor 32, thatpotential renders transistor 41 conductive which, in turn, permitscurrent flow through the charging circuit including the emitter-base oftransistor 41 and resistor 42 to charge capacitor 40. If the inputsignal maintains transistors 32 and 41 sufficiently long for thepotential across capacitor 40 to reach the threshold potential ofvoltage threshold circuit 45, the output of the threshold circuitbecomes zero to, in turn, render the power transistor conductive. Whentransistor 41 becomes non-conductive capacitor 40 rapidly discharges viadiode 43 and resistor 44. Thus, only input signals of sufficientduration to overcome the time delay can activate the power transistorand, hence, spurious noise signals of a lesser duration have no effecton the base drive circuit.

The output of voltage threshold circuit 45 is coupled to the base of aPNP type transistor 50 via a resistor 51. The base of transistor 50 isalso connected to the anode of a zener diode 52 with the cathode thereofconnected to the positive source. The emitter of transistor 50 isconnected to the positive source via a resistor 54 having a capacitor 55connected in parallel therewith. A resistor 53 is connected between theemitter and base of transistor 50.

If capacitor 55 were removed from circuit, the remaining componentswould function as a constant current regulator. The amount of currentflowing through transistor 50 is equal to the voltage across the zenerdiode minus the emitter-base voltage of the transistor as developedacross resistor 53 divided by the resistance value for resistance 54.When transistor 50 is initially rendered conductive by a positive signalapplied via resistor 51, capacitor 55 charges to provide a high initialcurrent flow through transistor 50 into the base of the powertransistor. The initial high current flow decays exponentially ascapacitor 55 charges and eventually settles at the constant currentvalue determined by zener diode 52.

Turn-off circuit 18 includes an NPN transistor 60 which couples the baseof the power transistor to the negative source to apply a continuousnegative voltage during the off condition. The collector of transistor60 is connected to the base of power transistor 10 via a resistor 61,whereas the emitter of transistor 60 is coupled directly to negativesource 21. The output of voltage threshold circuit 45, which controlsthe conductive state of power transistor, is coupled to the cathode of azener diode 62 with the anode thereof being coupled to the base oftransistor 60 via a resistor 63. A capacitor 64 is connected in parallelwith zener diode 62 and a diode 65 is connected in parallel withresistor 63. A resistor 66 is connected across the base emitter circuitof transistor 60.

Zener diode 62 provides a voltage offset to the base of transistor 60 sothat, when the output of voltage threshold circuit 45 is zero (thecondition which renders the power transistor 10 conductive), the zenerpotential maintains the base of transistor 60 at zero volts relative tothe emitter thereof so that transistor 60 remains non-conductive. Whenthe output of voltage threshold circuit rises to the positive value (thecondition which turns power transistor 10 off), the voltage offsetprovided by zener diode 62 and resistor 63 permits a similar rise in thepotential at the base of transistor 60 thereby rendering that transistorconductive. The series circuit including capacitor 64 and diode 65permits a faster transient response when transistor 60 is turned off toprovide a rapid turn-on for the power transistor.

The operation of the base drive circuit according to the invention canbest be understood in connection with FIG. 2 which is a curveillustrating the base current of the power transistor through a normaloperating cycle. Initially, there is no external signal applied to LED31 and therefore transistor 32 is non-conductive. The voltage thresholdcircuit 45 provides a positive output signal which maintains transistor50 of turn-on circuit 16 in the non-conductive state and causestransistor 60 in the turn-off circuit to conduct. Current thereforeflows from the base of power transistor 10 through resistor 61 and thecollector-emitter circuit of transistor 60 to the negative sourceprovided by power supply 21. The continuous reverse voltage from thephase of the power transistor maintains the power transistor in anon-conductive state. The reverse base drive provides protection againstspurious noise since any such noise signals must overcome the negativedrive on the phase in order to falsely trigger the power transistor intoa conductive state.

When an external signal is applied to LED 31, photo transistor 32conducts which in turn renders transistor 41 conductive to commencecharging a capacitor 40. If the signal applied to the LED remainssufficiently long, the charge on capacitor 40 builds up and exceeds thethreshold value of voltage threshold circuit 45. When the thresholdvalue is exceeded the output of circuit 45 drops to zero which in turnrenders transistor 60 non-conductive thereby terminating the negativebase. The zero potential at the output circuit 45 is coupled to the baseof transistor 50 and renders this transistor in turn-on circuit 16conductive. The charge of capacitor 50 provides a high initial currentat point 25 in FIG. 2. The current supplied to the base of transistor 10decays exponentially as capacitor 55 charges. After a short period t₁the value of the base current supplied to the power transistor reachesthe level 26 shown in FIG. 2 and is thereafter maintained at this levelby the constant current circuit configuration provided by zener diode 52in the emitter-base circuit of transistor 50.

Termination of the signal applied to LED 31 turns off the LED, renderstransistors 32 and 41 non-conductive and therefore capacitor 40discharges via diode 43 and resistor 44. The potential cross capacitor40 drops rapidly and therefore voltage thresholds circuit once againprovides a positive output signal which renders transistor 50nonconductive and renders transistor 60 conductive. When transistor 60becomes conductive, the base of the power transistor is connected to thenegative source providing a reverse current base drive which rapidlyclears out the carriers in the power transistor and assists in rapidlyturning the transistor off.

The constant current level 26 for the positive base drive which rendersthe power transistor conductive is selected to a value sufficiently highto maintain the power transistor in a fully conductive saturated statefor all anticipated load current conditions. The current regulator isset so that the base current is maintained at a maximum level below thatwhich would permit destruction of the power transistor in a fault statewhere a high current may be encountered. The fault state could include ashort or ground at the external load. By limiting the base current ofthe power transistor, the power dissipation in the conductive state islimited so that the power transistor is not destroyed.

Specific parameters for a specific power transistor namely theWestinghouse Electric Co. KD22457510 transistor are as follows:

regulated positive drive current=0.75 amperes

initial drive current=1.5 amperes

time interval t₁ =1 microsecond

negative drive current=4 amperes

While only one specific embodiment of the invention has been disclosedin detail, there obviously are numerous variations within the scope ofthis invention. The invention is more particularly defined in theappended claims.

I claim:
 1. A base drive circuit for a fast switching, power transistorcomprising:an input circuit responsive to external transistor turn-oncommands; a constant current regulator for normally supplying a basedrive to the power transistor in response to a turn-on command at alevel,sufficiently high to maintain the power transistor in the fullyconductive state for all anticipated currents, and below the level wherepower dissipation in the power transistor would be destructive in afault state, and; an override circuit coupled to said current regulatorto momentarily override said constant current to provide a high initialbase current to the power transistor to rapidly turn on the powertransistor, said high initial base current being for a durationsufficiently long that the current spreads evenly throughout the powertransistor.
 2. A base drive circuit according to claim 1 furthercomprising a turn-off circuit coupled to the base of the powertransistor to provide a reverse current whence the transistor isrendered non-conductive.
 3. A base drive circuit in accordance withclaim 1 wherein input circuit further includes a time relay circuit suchthat input signals of less than a predetermined duration are noteffective to turn-on said power transistor.
 4. A base drive circuit inaccordance with claim 3 wherein said input circuit further includes anoptical coupling circuit.